The present invention relates generally to magnetic resonance imaging. Magnetic resonance imaging (MRI) typically utilizes an RF transceiver and magnetic field gradient generation for spatial encoding. Various subsystems may be needed, such as magnetic field shimming and safety and the requisite software tools for signal and image processing. Encompassing all of this in a complex computer-controlled environment may require substantial effort. Although one can acquire a commercial spectrometer such as the Bruker AVANCE system, it is desirable to develop an improved system for reasons demonstrated below. The present invention relates to such a system.
One advantage to console development is the ability to incorporate new technology and solve problems in a timely fashion. For instance, known commercial spectrometer consoles may need development work in order to function with new technology. On the other hand, an improved MRI system may use newly introduced vendor-supplied devices for which the interfacing and control has not previously been incorporated into commercial spectrometer consoles.
The present invention allows experimentation at the lowest level in devising new strategies for maximum console performance. Multiple transceivers of the present invention may be used in a variety of ways: independent operation, multiple receivers using a single coil, multiple transmitters driving a single coil, etc. The gradient system of the present invention preferably supports large functions with fast and interactive adjustments. The preferred system data flow of the present invention will essentially remove image dimension barriers, and a new modular design of the present invention preferably lends itself to continuous improvement.
A spectrometer requires a receiver to convert magnetic resonance (MR) signals captured by a coil to digital data suitable for image rendering and spectral analysis. A transmitter is required to generate RF excitation signals from digital data. Because the function and design of the receiver and transmitter are closely related, they are referred to collectively as a transceiver. Known references that describe fundamental transceiver requirements deal primarily with MRI at field strengths of 3 T or less.
In UHFMRI (e.g., 8 T), the main challenge continues to be receiver design; specifically, reconciling the mutually exclusive goals of sensitivity, high dynamic range, and wide bandwidth. Sensitivity is an issue because MR signals are very weak, and therefore arrive at the receiver with low signal-to-noise ratio (S/N). Dynamic range emerges as an issue because it is often desired to observe very weak signals (such as spectral lines) in the presence of signals that can be about 110 dB stronger. A desire for increased bandwidth, up to about 1.5 MHz, arises from the extraordinary gradient strengths and performance available on a state-of-the-art 8 T system. However, the required sample rate increases with bandwidth, and the maximum bandwidth is not always required. Thus, the ability to vary the bandwidth over the range from about 2 kHz to about 1.5 MHz is desirable. Furthermore, it is preferred that the MRI transceiver is adapted to accommodate a wide frequency range, e.g., from about 10 MHz to about 400 MHz.
The design challenges for a UHFMRI transmitter include two of the key problems in receiver design: wide tuning range and image rejection. Image rejection is a significant consideration for the transmitter design because image signals from the transmitter will actually be radiated from the coil.
Because resonant frequency is proportional to magnetic field strength, transceivers for UHFMRI preferably accommodate a wider tuning range than traditional MRI transceivers. Resonant frequencies for some key isotopes at 1 T and 8 T are shown in Table 1. This table suggests a preferred minimum tuning range specification of about 30 MHz to about 350 MHz for an 8 T MRI.
TABLE 1 MR frequencies for key isotopes at 1 T and 8 T Isotop Resonance at Resonance at e 1 T 8 T .sup.1 H 42.577 MHz 340.616 MHz .sup.19 F 40.055 320.440 .sup.31 p 17.235 137.880 .sup.7 Li 16.547 132.376 .sup.11 B 13.660 109.280 .sup.23 Na 11.262 90.096 .sup.27 M 11.094 88.752 .sup.13 C 10.705 85.640 .sup.2 H 6.536 52.280 .sup.17 O 5.772 46.174 .sup.15 N 4.315 34.520
The purpose of an MR receiver is to produce a high-fidelity baseband digital representation of the MR signal suitable for computer analysis. Known analog-to-digital converters (A/Ds) cannot directly digitize signals at the frequencies associated with mid to ultra-high-field MR, so frequency downconversion is currently necessary. Downconversion involves mixing the MR signal with a local oscillator (LO) to produce sum and difference frequencies. Given an appropriate LO, the difference frequency (sometimes also called the intermediate frequency or "IF") will be low enough for digitization with known A/Ds. Because the performance of an A/D degrades with increasing frequency, there is an incentive to make the difference frequency as low as possible.
A fundamental problem in this method of downconversion is image rejection. For example, suppose that a "low-side" LO is used, i.e., the LO frequency f.sub.LO is less than the center frequency of the desired MR signal f.sub.MR. In other words, f.sub.MR =f.sub.LO +f.sub.IF, where f.sub.IF is the difference frequency. In this case, the receiver is also sensitive to the image frequency which is f.sub.LO -f.sub.IF. Any signals present in the image band will appear at the output as if they were originally in the desired sideband. Even if no signals are present in the image band, the image band noise appears at the output of the mixer and adds with the desired signal, thereby degrading S/N. This problem can be addressed by filtering the image band before mixing. However, this is difficult to do if f.sub.IF is small. This provides incentive for increasing the difference frequency, which runs counter to A/D capabilities.
Image rejection by filtering before mixing has other problems as well. One is that tuning the receiver becomes difficult because the image filter must also be tuned. Thus, an image rejection architecture may be helpful. Image-rejection downconversion schemes (also known as "single sideband" or "SSB" demodulators) are helpful in eliminating image energy over a wide tuning range without the need for tunable image filters. FIG. 1 shows a known image rejection scheme in which in-phase (I) and quadrature (Q) local oscillator (LO) signals are applied to the input signal, resulting in audio-frequency I and Q channels that contain both the desired sideband and the image. The image sideband can be canceled by shifting the phase of the I channel by .+-.90.degree. (depending on which sideband is desired), and then summing the result to obtain a real signal. The 90.degree. phase shift has the effect of putting the desired sideband in the I and Q channels in phase, and the undesired sideband in the I and Q channels 180.degree. out of phase. Thus, the sum signal contains only the desired sideband.
In the MR field, a 40 dB image rejection in a 100 kHz bandwidth has been obtained using analog filters to implement the phase shift. In this scheme, the sources of error included amplitude and phase imbalance in the analog I/Q demodulator, and phase variation with frequency due to the audio phase shift circuit. It is particularly difficult to construct audio phase shift circuits that are invariant with frequency. This is primarily because phase shifts in real signals must be implemented as delays, and it is difficult to realize a filter with delay that varies precisely with frequency. As the fractional bandwidth (that is, the ratio of the bandwidth to the center (difference) frequency f.sub.IF) becomes larger, this technique becomes more difficult.
For wideband MR signals, there are two ways to improve image rejection. One is to increase f.sub.IF, reducing the fractional bandwidth and thereby simplifying the filter design. The other is to digitize the baseband I and Q channels and correct the amplitude and phase errors incurred in the analog I/Q demodulation using digital signal processing techniques. This can be computationally expensive, especially for high sample rates. The former suggestion also poses a problem because A/D performance generally degrades with increasing frequency and sample rate.
In light of the shortcomings of the known art, a need exists for a receiver and a transmitter that are adapted to work with MRI at field strengths up to about 8 T. Another need exists for a receiver that has improved capacity to work with MR signals that have a low S/N. A need also exists for a receiver that has improved dynamic range. Yet another need exists for a receiver and a transmitter that enable the bandwith to be varied over the range from about 2 kHz to about 1.5 MHz. Still another needs exists for a receiver and transmitter that can accommodate a frequency range from about 10 MHz to about 400 MHz. Finally, a need exists for a receiver and a transmitter that have improved image rejection.